Building a 6G3 Based Amplifier
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I wanted something different from my modified 1980 volume-boost Princeton Reverb, perhaps with a little darker, compressed tone. I am going to build an amplifier based on the tweed Fender Deluxe, probably a 5E3 circuit, as a test bed for modifications. But to use as a contrast to the PR, a brownface Deluxe seemed to be the ticket.

I started with a kit from Mojo which includes generally good parts. One criticism of this kit it that it uses carbon composition resistors throughout which imparts a bit more hiss to the amplifier than, say, metal film. Sonic cork-sniffers may complain that those resistors change the sound and are not "vintage" but neither are modern carbon composition resistors. They definitely have less hiss, better consistency, better temperature coefficients, and more stability than the resistors manufactured in the 50s. Still, they are noisier than modern metal film/foil.

First step is the filter caps which are in the doghouse underneath the chassis. The kit included a carbon composition 25kΩ resistor. Generally a metal oxide it preferred in the power supply for less fire risk.

The image does not show the final version with individual leads off the negative poles of the 3rd cap and of the 4th cap which go the separate ground points. The picture does not show the 220kΩ bleeder resistor across the first cap.

The wires are routed into the chassis. The 3/8" hole for the bias pot is not shown; that had yet to be drilled.

The power transformer (Classic Tone 40-18028) is installed. The V6 socket for the rectifier tube was wired, including so-called "back-up diodes" from pin 3 -> 4 and 5 -> 6. These pre-rectify the input voltage and protect the amplifier if the rectifier was to fail shorted. They also should protect the rectifier from being impacted by peak inverse voltage exceeding it's rating and should increase the lifespan of the tube.

You also see the voltage divider (consisting of a 270kΩ resistor, a 27kΩ resistor, and a 100uF 100V capacitor) which was later installed to elevate the heater center tap to 38V, reducing 60Hz hum.

Here's the bias board, after installing the circuit board and wiring it up.

Here's right side of the nearly finished chassis. Notice the buss ground, which is grounded to the chassis. This had to be revised as it resulted in mild 120Hz hum. What I did was to group all the grounds by power supply node. That is, the last filter cap feeds V1 A&B and V2A. Those cathodes and the grounds for the volume and tone pots for each channel were tied to the negative lead from the filter cap and grounded together. The 3rd filter cap supllies V3, the PI. The negative lead of the 3rd filter cap was brought out and grounded with the cathodes of the PI circuit. The grounds of the tremolo (V2B cathode and grounds of the speed and intensity pots), the bias supply, and the 1st and 2nd filter caps were all grounded with the cathodes of the power tubes and PT center tap on a mounting bolt of the PT. The ground of the speaker jacks and tremolo pedal jack were also grounded here.

This amp also includes a hi/lo power switch copied from my Fender EC Tremolux. See this description. Here you can see the chassis-mounted 8Ω and 16Ω resistors.

Posted by Gordon, No Hair Blog, Apr 18, 2018

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